1. Field of the Invention
The present invention relates to anisotropic conductive adhesive used when connecting a flexible wiring board to a panel electrode of a liquid crystal display unit, for example, and further to a connection structure and connection method of the electrode using the anisotropic conductive adhesive.
2. Description of the Prior Art
Since a liquid crystal display unit, for example, is a planar display unit having excellent properties, such as small thickness, light weight, low power consumption, etc., it is put to a wide range of applications to mobile devices, such as a Personal Digital Assistance (so-called PDA), a portable telephone, etc., to the display of a personal computer, etc.
The liquid crystal display unit has a liquid crystal panel of a structure having a liquid crystal layer sandwiched between a pair of display panel substrates, i.e. an array substrate and an opposed substrate, and controlled by applying voltage selectively to pixels between the array substrate and the opposed substrate to display images on the liquid crystal panel. In an active matrix liquid crystal display panel, for example, an amorphous silicon or polysilicon semiconductor is used for an array substrate to form a Thin-Film Transistor (TFT) as a switching device and simultaneously form pixel electrodes, scan lines, signal lines and so on connected to the switching device. On the other hand, an opposed substrate is formed with opposed electrodes of Indium Tin Oxide (ITO) or the like, color filters, etc.
In the liquid crystal display unit of the structure mentioned above, an external control circuit that feeds display signals is disposed on a circuit board independently of the liquid crystal panel and, for example, a flexible wiring substrate is generally utilized as a lead wire to electrically connect the external control circuit to a circuit formed on the liquid crystal display panel. In this case, since it is required to electrically and mechanically connect an electrode formed on the flexible wiring board to a panel electrode formed on the liquid crystal panel, anisotropic conductive adhesive has widely been used for this connection.
In the aforementioned liquid crystal display unit, with an aim to reduce the weight and thickness thereof, the array substrate has a scan line-operating circuit and a signal line-operating circuit built-in. Particularly, since the signal line-operating circuit has to be operated at higher speed than the scan line-operating circuit, it is formed as an IC chip, for example, and the IC chip is mounted on the array substrate using the Chip On Glass (COG) technique. The IC chip is mounted as being connected onto a panel electrode provided on the outer edge of the array substrate via a bump, receives signals output from an external control circuit and inputs image-controlling signals to the scan lines and signal lines of the array substrate. When mounting this IC chip, anisotropic conductive adhesive has also been used for connection.
The anisotropic conductive adhesive is adhesive having conductive particles dispersed therein. The conductive particles get squashed between the electrodes by pressure application to establish electric conduction between the electrodes. As disclosed in JP-A HEI 10-274778, resin particles plated with metal are used as the conductive particles. The invention of the prior art relates to an electrooptic apparatus equipped with a drive circuit chip attached onto a part-attaching portion via anisotropic conductive adhesive and with a film circuit board. The anisotropic conductive adhesive includes conductive particles having elasticity that comprise elastically deformable resin particles plated with a conductive metal film.
In the case of using the anisotropic conductive adhesive to connect two electrodes, no problem on the connection state will arise when the opposed surfaces of the electrodes are flat. However, when the surface of one of the electrodes is concavoconvex, for example, complete connection between them is not always acquired, resulting in possibly raising problems, such as a rise in connection resistance etc. Such problems are liable to arise particularly when the degree of concavoconvex is large. This is because pressure is difficult to apply to the conductive particles on the concave portions while pressure is infallibly applied to the conductive particles on the convex portions. This difference in pressure applied produces portions where the conductive particles get strongly crushed (convex portions) and portions where they get weakly crushed (concave portions). Sufficient repulsion cannot be obtained at the weakly crushed portions to impair the state of contact with the electrodes, thereby giving birth to a rise in connection resistance. To solve the problem, it is conceivable that the electrodes are pressed against each other so that sufficient pressure may be applied to the conductive particles on the concave portions. In this case, however, the conductive particles on the convex portions will collapse due to the excessive pressing force, leading to failure to acquire sufficient electrical conduction at the convex portions.
The present invention has been proposed in view of the conventional state of affairs described above. An object of the present invention is to provide anisotropic conductive adhesive by which the state of connection between electrodes and conductive particles at both concave and convex portions where the surfaces of the electrode is concavoconvex, is kept good without any rise in connection resistance and therefore to provide a connection structure and a connection method capable of realizing connection low in connection resistance between electrodes and excellent in reliability.